On 2019-02-14 6:37 a.m., André Malm wrote:

> 
> 
> The 'out' file is 34M, which is the full size.
> 
> What causes this?
> 

Just to narrow it down, the fault is the contents of the server.jar
file, not how it was created.

If I upload a urandom server.jar file to my own website, then download
it with wget as described, btrfs send creates a properly small diff

However, if I upload the actual server.jar file to the same URL, then
repeat the process, I again get a 34MB out

How often does this happen?  Did you just happen to stumble on the one
file in the Universe that does this with server.jar?  Or was there are a
wider context?